In my experience, pipeline/forecast is one area of a CRM system that changes frequently. Fortunately, these changes require less coding changes through Windows Workflow Foundation (Dynamics CRM) and Workflow Automation (Salesforce). I have seen this both from the consulting side and as a user. Not going to comment on whether it is right or wrong, just that it happens, often, as a matter of fact in many organizations. Change in management, there will be a change to the sales process. New compensation model and the forecasts will change. It happens all the time and makes numbers from sales very short term and difficult to do long term trends. The best way to improve accuracy, find a sales process that works and then measure the core of that process. Less change, more value.